1. Field of the Invention
The present invention relates to an apparatus for selecting any one of a plurality of video input signals. Such an apparatus is of particular use when is it desired to select video signals from a plurality of video tape recorders (VTR's).
2. Description of the Prior Art
A problem with the selection of video signals from several VTR's is that such machines are relatively unstable when compared with purely electronic video equipment, such as video cameras, vision mixers, and sync pulse generators (SPG's). Because VTR's are electro mechanical machines, the timing of the video signals reproduced therefrom varies, typically by a period of several lines of video signal, despite the fact that the VTR's are synchronized to the electronic video equipment in a television studio or the like. In order to overcome this problem, it is known to provide each VTR with a time base corrector (TBC). A TBC is an apparatus which essentially comprises a memory in which the video signal is written in synchronization with the VTR to which it is connected and from which the video signal is read in synchronization with the sync pulse generator (SPG) of the television studio or the like. Modern TBC's employ digital electronics and have a random access memory, an analog-to-digital convertor, and a digital-to-analog convertor. The analog-to-digital convertor and the write clock of the memory are referenced to the video signal received from the VTR whereas the digital-to-analog convertor and the read clock input of the memory are referenced to the SPG of the television studio. Thus, the output video signal from the TBC can be accurately synchronized to video signals from video cameras and other VTR's provided with respective TBC's, so that switching between the various sources, which is normally carried out during the vertical blanking period, can be performed without any loss of synchronization or break-up of the video signal.
A disadvantage of this known arrangement is that TBC's are very expensive and can cost as much as or more than the VTR's to which they are connected. This is a substantial disadvantage when it is desired to select between several VTR's, as is nowadays common practice in television studios and the like.
British Pat. Nos. 1 513 467, 1 576 623, and 2 026 278 B, published before the priority date of the present application, and British Patent Specification No. 2119597 A, published after the priority date of the present application, disclose various arrangements for providing synchronisation. However, these arrangements are relatively complex and/or unsuitable for use in a profession studio environment.